1. Field of the Invention
The field of the invention is subterranean well completions, or, more specifically, methods and apparatus for independently completing target completion intervals in a cased wellbore without conventional perforations.
2. Description of Related Art
In the oil and gas industry, many wells have more than one subterranean interval targeted for completion. In particular, wellbores frequently have a horizontal portion deviated from a generally vertical well into a large, productive formation. The horizontal portion extends deep into the formation, in a direction generally perpendicular to the vertical wellbore. It is typical for the operator to have predetermined which intervals along the horizontal wellbore are likely to provide the optimum production of the formation. In such a case, the operator desires that the intervals be initially accessed and treated in isolation from other intervals. To this end, it is currently the most common practice to run casing through the horizontal wellbore, cement the casing in place, perforate the casing proximate the first and lowest desired interval, treat the perforated interval (such as with a frac job using frac sand or ceramic spheres as proppants), isolate the interval using bridge plugs or sand plugs, then perforate and treat and isolate the next interval, repeating until all desired intervals have been perforated and treated. This practice requires that a time-consuming trip out of the hole and back in be made with all the tools between each interval.
Another practice includes utilizing a cutting/washing tool to cut two holes in the casing proximate the desired completion interval using special sand-water mixture jetted against the casing from the tool, washing out the sand from the wellbore, then pulling the cutting/washing tool into the vertical casing, treating the newly exposed formation interval through the two holes, and then isolating the newly treated interval by placing a sand plug at and above the two holes. Among the problems with this approach is the inability to determine the precise location and size of the two holes, which are typically small, with a combined area of far less than the cross-sectional area of the casing bore. This large difference in areas has a “choke” effect on the total fluid flow, such that the full casing bore capacity is underutilized. An additional problem is the special cutting sand, of which a notable amount remains in the wellbore in the vicinity of the two holes, even after washing. It is believed that this special cutting sand is carried into the formation by the initial fluids used in the following treatment (frac fluids), and often completely plugs formations near the holes, thus “screening out” the injection and preventing the completion of the frac job. This plugging effect is different from the usually benign practice of spotting normal frac sand in the casing to a point above the two holes—such sand being efficiently washed from the holes without a post-completion plugging effect. Typically, this practice has proven to be unnecessarily time-consuming and will typically result in one interval completion per day, occasionally going as high as three intervals. completed per day.
Attempts have also been made to use flapper valves to isolate the interval below while perforating and treating the interval above. These attempts have been only partially successful, largely due to the nature of mechanical shifting tools, which open once and stay open, and the mechanical shifting tools failing to properly close the flapper valves, as well as, sand interference in the flapper mechanism itself, or sand interference between the shifting tool and the flapper valve profile that interacts with the shifting tool. Attempts include wells where conventional perforation were used, as well as, wells where the perforations were cut as holes, in the manner described above. When the perforations are cut as holes, the sand used for cutting holes is known to exacerbate this problem.
In still another practice, sliding sleeve tools have been included on the casing proximate each of the desired intervals, with the sliding sleeve intended to move to a position that opens slots to allow formation interval fluids to enter through the sliding sleeve. In this practice, the sliding sleeve was re-closed with a second shifting tool, in order to isolate the newly treated interval from the next highest interval, prior to opening the sliding sleeve for the next highest interval. The sliding sleeve tool was of the type having sleeve holes alignable with surface holes after rotation of the sleeve. Although this method has been successfully applied in completions in relatively shallow wells, where frac jobs are conducted at relatively low pressures and proppant concentrations (e.g. 1-2 ppg), this practice has not been successful in deeper wells, where frac jobs are conducted at much higher pressures and proppant concentrations (e.g. 6-7 ppg). The increased concentrations of proppant cut the sleeve in the sliding sleeve tool causing an inability to re-close the opened tool that is in the lowest completion interval, and an inability to open the sliding sleeve tools in the higher completion intervals.
What is needed is a downhole tool for use in lieu of conventional perforations and perforation cutting, particularly in deep wells where treatment will involve frac jobs with high proppant concentrations, is reliably positionable and openable when run as part of a casing string and placed proximate the desired completion intervals in a horizontal wellbore. Also needed is a reliable and cooperative means for isolating the treated intervals, while opening and treating higher intervals.